Soft rot is a plant disease which is caused by Erwinia carotovora and causes plant tissues to become soft and rotten. Examples of a plant to be a target for controlling soft rot include vegetables such as cabbage, celery, Japanese radish, tobacco, onion, tomato, carrot, Chinese cabbage, potato, lettuce, wasabi, and the like. Since a pesticide is rarely effective in a bacterial disease, an antibiotic such as streptomycin or a copper-based bactericide as described in Patent reference 1 is conventionally applied to a target plant to effect disease control. However, an antibiotic which has been used for controlling a bacterial disease such as soft rot is expensive and also has a problem that resistant bacteria appear. Further, in the case of using a copper-based bactericide, there are problems that (1) when the air temperature is high, phytotoxicity occurs; (2) a disease control effect is not obtained unless a large amount of the bactericide is used, and it raises a concern that metal contamination occurs; and the like. Therefore, the development of a disease control method which solves the problems of the antibiotic and the copper-based pesticide has been demanded.